Where to start. Well. This information is not exactly easy to decipher for a beginner. Let's assume you had the presence of mind to measure your existing shaft BEFORE you blew it all to pieces plowing or 4 wheeling. You did, right? If not, I'll cover picking up the pieces on that later. So, you have a length, and a spline count. You may even have the bolt pattern needed - there are several. The common ones are 8 x 3.96/3.97" which is what I'll be covering here.
The first step is to look through the full float rear axleshaft table in Dana Spicer XGI catalog. In my copy, the table starts on page 135 of the PDF. It is very helpfully sorted by length, so just use the search tool in your PDF app to highlight the spline count you need. In my case, I entered "1.50 - 35". Scroll until you find the right length (you'll want to find one slightly longer or equal length, you can always shave it down about 1/8" if needed) and verify the bolt circle, bolt count, and flange diameter are acceptable. Write down the part number.
Now begins the annoying part. You'll be looking through up to half a dozen (or so) Dana Spicer X510 catalogs to find what make/model/year the correct BOM axles came from. I recommend starting an excel spreadsheet to keep track of all this because it is going to get away from you quickly, it's a lot of info. Search the catalogs, write down all the axle BOMs and which side the shaft fits, then use your favorite junkyard search tools (Car-Part, Hollander, row52.com) to find possible donor vehicles. Hit the yard and check the axle tags before you even bother to turn a wrench.
Cliff notes for which X510 catalog has what in it:All of these catalogs can be found online in various places via Google easily enough, but for my own use (and your convenience) I've copied them to this website at this location. If it starts eating up too much of my bandwidth I'll probably dump it again and make you use google, but they're here for the moment.
Most of those catalogs are of interest only to the curious. The ones you should focus on for dana 60 and larger info are the -8, -9, -2, and -5 catalogs. Honestly, you can probably just stick to the first 3, but might as well check the -5 as well. Happy junkyard hunting!